My son-in-law and I each have a Polaris scratch built foamie. I just finished mine and before flying (which I did not do) I did some "sailing" on the water. Close by, up to about 100 feet control was good. As I went further away the plane became hard to control and a about 200 feet away there was a several second delay on control response (rudder and throttle). Tried my son-in-law's transmitter and had the same result. Replaced the receiver with a new one and had the same problem using both transmitters. Both of our xmitters worked on his plane. Both our planes have ESCs with the built in BEC. Could my ESC be bad?

Moved to electric aircraft forum

The esc is not the problem.
What radio and receiver are you using?
My guess is the placement of the receiver and antenna inside the airframe. If the receiver and antenna are at or below the waterline you will loose or have diminished radio control the farther out you go on the water.
Try relocating the receiver so you can get at least the antenna higher up in the airframe.

X2. Perform a range check on the ground, then do one in the water and it should confirm your problem is due to rx placement.

You can also have problems with reflection off waves. Using circular polarized antennae is preferable for water crafts for that very reason. All reflections are out of phase with circular poarization so do not diminish signal strength at the receiver.

Radio is a 9XR Pro using the Fly Sky 2.4GMH module (model FS-RM002). Receiver and antenna are as high above the water line as possible. We both are using this system. Using his tx and rx in my plane gives the same result. Placement in both planes is the same. We did not swap batterys or ESC. His is a 40amp with BEC built in. Mine is a 35 amp with BEC included. I am starting to suspect the BEC is overloaded.

Turnigy ? Fly Sky? Is the esc and motor Turnigy as well ? I have little faith in Turnigy gear.
You stated you were loosing control but you didn't loose it completely for 10-20 sec?
If the bec is not providing enough current under load then the servos would be sluggish but this would happen on the bench.
If the bec was not providing enough voltage then the receiver will/should brown-out and you loose control completely until the bec resets and then resets the receiver(several seconds) ....but again this should happen on the bench under a load test. Could be a bad motor or servos, or wiring too but this should all prove itself on the bench.
Loss of radio control at range is typically a radio/receiver issue.
If you can't trouble shoot by testing then you will have to start swapping components until the problem is corrected. Wiring, motor, esc, servos, etc...